MICROSCOPIC TECHNIQUE. 19 



the whole with a dry blotting paper. The section will now adhere 

 and the paper may be removed. Now cover the section with a thin 

 layer of collodion mixture, and it will be thoroughly affixed. 



Centering. The process of fixation should also include the cen- 

 tering of the section. This may readily be accomplished by means 

 of a diagram of the slide, which may be drawn upon the under sur- 

 face of the cover of a mailing box. See laboratory exercise No. 3. 



7. Staining. This process consists in tinting the structures of the 

 section with certain stains, so as to produce a differentiation of the 

 different elements and render them more readily studied. It de- 

 pends upon the principle that certain structures have an affinity for 

 certain stains, but not for others. For example, a stain that will 

 affect the protoplasm and nucleus may have no effect upon the cell- 

 wall. A stain that will affect certain cells may have no effect upon 

 others. Haematoxylin will stain the leucocytes, but not the red cor- 

 puscles. Eosin will stain the red corpuscles, but not the white. 

 Staining should always be preceded by certain processes so as to 

 prepare the tissue to receive it. First, if a paraffin section is to be 

 stained, the paraffin must be removed. This is accomplished by 

 immersing it in xylol, turpentine, chloroform, or benzole. These 

 reagents dissolve out the paraffin. The xylol, etc., may then be re* 

 moved by applying alcohol. Should an aqueous solution of any 

 stain be used, the section should be washed with water before ap- 

 plying the stain, and followed with the same. If the stain be alco- 

 holic, its application should be preceded and followed by alcohol of 

 the same strength. The different staining methods are given on 

 page 23. 



8. Dehydrating. This consists in the removal of water from the 

 specimen, water being generally the enemy of the histologist. It 

 is accomplished by running the section through increasing strengths 

 of alcohol, using first an alcohol of the same strength as the stain- 

 ing solution. The object of this is to prevent the precipitation of the 

 stain, by which the preparation becomes filled with dark granular 

 masses. 



9. Clearing. This consists in the removal of the alcohol so as to 

 prepare the sections for balsam and in so clearing up the section as 

 to render it transparent. By closely observing the change in color 



